Skeptics often say they are trying to expose pseudoscience, but in reality we tend to use this term loosely. Creationism, homeopathy, al­ternative medicine, and cold fusion are clearly pseudoscientific, but what about ancient aliens, UFOs, alien abductions, Bigfoot, crystals, the Moon hoax, and many other claims investigated in the pages of the Skeptical Inquirer? Are these examples of pseudoscience, just bad science, or perhaps not related to science at all?

One definition of pseudoscience presents it as claims that are presented as scientific but do not adhere to valid scientific method. Another describes pseudoscience as the misuse of methods that seem scientific in order to undercut real science. In his excellent new book The Pseudoscience Wars: Immanuel Velikovsky and the Birth of the Modern Fringe, Princeton historian Michael D. Gordin asserts that scientists only apply the label of pseudoscience—which he defines as “doctrines that are non-science but pretend to be, or aspire to be, or are simply mistaken for scientific”—to ideas that they feel are threatening because of their public appeal. It is the way pseudoscience masquerades as real science and is used to attack real science that sets it apart from the easily dismissed claims of cranks and charlatans.

No advocate for an unorthodox perspective ever calls his or her work pseudoscience. This is a term of opprobrium, assigned by scientists who are defending the consensus. Gordin notes that in practice, the term pseudoscience is generally reserved for ideas that are perceived as major challenges to science—especially in the eyes of the public. Less threatening ideas are simply labeled as bad science or non-science, on the assumption that they will self-destruct and be quickly forgotten.

Gordin’s highly readable book ex­amines in detail several twentieth-century examples of pseudoscience. Major emphasis is on the pseudocosmologist Immanuel Velikovsky. His other prime examples are Trofim Lysenko, who nearly destroyed mid-century Russian genetics, and the advocates for Biblical creationism who attempted to establish a scientific basis for a planet less than ten thousand years old. Lysenkoism was a threat be­cause of the personal support given by Stalin and the Soviet Communist Party, which virtually outlawed research that challenged Lysenko and thereby had a strong negative impact on Soviet agriculture. In a sense, Lysenko could be held responsible for the death of millions in the U.S.S.R. The creationist authors who developed “flood geology,” primarily George McCready Price, Henry Morris, and John Whitcomb, have been (and continue to be) a threat because of support by Christian fundamentalists who use these ideas to attack evolutionary biology in particular and science in general. Velikovsky is different, in that he achieved substantial fame and influence on his own merits without the support of any organized religious or political institutions.

Many readers of the Skeptical Inquirer are familiar with the outlines of Velikovsky’s rise and fall. Gordin had access to the complete Velikovsky archives held by Princeton University, which he used to document many details of this story. He places in context the “Velikovsky affair,” which dealt with the opposition to his 1950 book Worlds in Collision. One of many controversies he illuminates is the question whether Velikovsky’s book was reviewed (or refereed) before publication. The fact is that it was reviewed by several scientists who said, in effect, even though it was bunk as science, the book was still likely to be popular enough that its publication would be a sound business decision.

That is exactly what happened. Scien­tists excoriated the book for its obvious failings, but many non-scientist reviewers lavished praise upon it. This deep division between the “two cultures” of science and the humanities came as a shock to many scientists. Velikovsky, while of course pleased that his book rose to the top of the bestseller lists, was also stung by its unanimous condemnation by scientists. He never understood their criticism of planets rapidly shifting orbits within historic time, and he sought their recognition as a bona fide scholar. He did not do this by publishing papers or making presentations at scientific conferences, but rather by courting individual scientists—especially his Princeton neighbor Albert Einstein. These efforts at personal diplomacy were not successful. Scientists immediately recognized that his planetary collisions, and his suggestion that electromagnetism rather than gravitation dominated planetary motions, were (in Einstein’s terms) “crazy.” Velikovsky was simply not speaking the same language as the scientific community.

But this is only half the story. It seemed likely that for all the immediate popularity of his book, he would quickly be forgotten. What makes this story so interesting, what elevates Velikovsky to the rank of true pseudoscientist, was the resurrection of interest in his work during the Vietnam War years, when he became a darling of the counterculture, gaining support among humanist scholars as well as rebellious students. He was invited to speak on campuses, conferences were held to discuss his ideas, and his books again ascended the best-seller charts. Precisely because of his rejection by the scientific community, he became the symbol of the lone scholar defying the conservative establishment.

Gordin also explores the relationships between different pseudoscientists. Superficially, there were striking similarities between Velikovsky and the creationist positions of Price in The New Geology (1923) and Whitcomb and Morris in The Genesis Flood (1961). All rejected the uniformitarian geology of their time. All sought to explain many geological features in terms of recent catastrophic events. And both sides sought evidence to support or verify events in the Old Testament, such as the Noachian flood and the escape of the Israelites from Egypt. Velikovsky began correspondence with Price in 1951, and he sent the manuscript of his second book, Earth in Upheaval, to Price for his comments. Price replied that “While I have not always been able to agree with some of the details. I have admired the handsome way in which you have demolished Charles Lyell as well as [Charles Darwin].” Later in a review, Price described “Earth in Upheaval as one of the most thought-provoking books of modern times.” However, Velikovsky and the creationists soon fell into dispute. Velikovsky sought naturalistic explanations for the Bible stories, while the creationists preferred direct attribution to acts of God. In the end, each side ostracized the other and went its own way, not acknowledging any commonality of ideas.

In his final chapter, Gordin turns to the new phase of pseudoscience, practiced by a few rogue scientists themselves. Climate change denialism is the prime example, where a handful of scientists, allied with an effective PR machine, are publicly challenging the scientific consensus that global warming is real and is due primarily to human consumption of fossil fuels. Scientists have watched in disbelief that as the evidence for global warming has become ever more solid, the deniers have been increasingly successful in the public and political arena. One has only to attend a meeting of atmospheric and climate scientists, such as the December 2012 American Geophysical Union, to appreciate the overwhelming support and increasing sophistication of our understating of human-caused global warming. At this gathering, thousands of scientists made hundreds of presentations of research results, ranging from the minutia of modeling the feedback of cloud formation on the greenhouse effect to documenting the incredible rate of loss of Arctic ice. Leaders of the scientific community made impassioned statements about the threat we face and the necessity for action. Yet outside the halls of science, polls show that half of Americans deny the reality of climate change, while Senator Jim Inhofe recently announced not only that human induced climate change is a hoax, but also boasted, “we have won” in the court of public opinion. Today pseudoscience is still with us, and is as dangerous a challenge to science as it ever was in the past.

Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe. This interdisciplinary field encompasses the search for habitable environments in our solar system and habitable exoplanets outside our solar system, the study of prebiotic chemistry, laboratory and field research into the origins and early evolution of life on Earth, and studies of the potential for life to adapt to challenges on Earth and in outer space. Astrobiology ad­dresses the questions of whether life exists beyond Earth and how humans can detect it if it does.

Astrobiology grew out of a previous discipline called “exobiology,” and often the two terms are used interchangeably. Exo­biology, however, is somewhat narrower—it focuses on the search for life beyond Earth and the effects of extraterrestrial environments on living things. The transition to the term astrobiology at NASA in the mid-1990s was a response to several discoveries that suggested that the space exploration program might help provide answers to fundamental questions about the origin, distribution, and future of life on Earth as well as beyond. These events included the evidence that Mars rock ALH84001 might contain microbial fossils, the amazing life found deep in the ocean at hydrothermal vents, recognition that Jupiter’s moon Europa had a vast ocean of liquid water, and the discovery of the first planets orbiting other stars. Today astrobiology has become a major area of space research globally, although we have not yet found evidence of life on other worlds.

Question: How could we find life on other planets? Does life in the universe have to be rare because life requires very specific environments?

Never having found any life outside the Earth, we cannot answer the question of whether life is abundant or rare. However, recent research with the Kepler mission and other astronomical observations suggests that habitable planets are common, with Earth-sized planets within the habitable zone orbiting at least 1 percent of stars. Even if only one-tenth of these planets actually support life, there are still roughly a billion living planets in our galaxy alone. We have no reason to think that our own planet Earth is unique or even unusual among the rocky worlds orbiting distant stars. But until we have data, this is just speculation.

There are three ways that we might discover life on other worlds: (1) Within our solar system, where we have direct access through spacecraft missions. We are especially interested in Mars, Jupiter’s moon Europa, and Saturn’s moon Titan. It is possible to search directly for microbial life on these worlds. (2) If abundant microbial life exists on an Earth-like planet orbiting another star, it is likely to change the atmosphere in ways that can be detected using large telescopes. On Earth, life has contributed oxygen and methane to our atmosphere, both “biosignature gases” that can be detected remotely. (3) Finally, if an intelligent, technological civilization has developed on any planet, and if “they” choose to broadcast radio or optical signals, then perhaps these signals could be detected by SETI searches.

Question: Why is the U.S. government building bunkers to house the elite in case of a global disaster? Also I heard about two suns in China. Is one of them Nibiru?

These two questions both illustrate the Big Lie, which can happen when a story goes viral on the Internet. Both are old hoaxes that just won’t go away. The government bunker rumor has two sources. First is the well-known fact that the U.S. government built many deep air raid shelters during the early stages of the Cold War, including the underground complex at Greenbrier, West Vir­ginia, which was supposed to provide protection for the President and his staff, and the Cheyenne Mountain complex in Colorado, which is still used by the Strategic Air Com­mand. Second is an episode on bunkers in Jesse Ventura’s TV series Con­spiracy Theory. Unfortunately, many people don’t realize this is an entertainment show, not journalism—something that is given away by the title, since the phrase “conspiracy theory” implies an interpretation that is not based on facts. But even his episode about shelters is all innuendo; Ventura does not actually interview anyone who claims to have seen one of the phantom “government bunkers.”

The source of the “two suns in China” report is even less well understood by the public. If you watch the original news video with a friend who speaks Chinese, you will see that this is about a single photo, broadcast by a local TV station on a small island near Taiwan. It is based on a still picture (not a video), apparently rephotographed with a hand-held video camera. As far as I can tell, the “two suns” were not seen elsewhere in Taiwan, let alone across China. And obviously if this phenomenon were real, billions of people everywhere could have verified it just by looking up at the Sun.

Question: Is it true that the reason the far side of the Moon has more impact craters than the near side is because the Earth shields the near side from incoming comets and asteroids?

The Earth shield is not large enough to in­fluence crater densities. Just using simple straight-line geometry, you can calculate how much of the lunar sky is obscured by the Earth: about four square degrees out of 41,000 square degrees for the whole sky. Only one in ten thousand incoming projectiles would be intercepted by the Earth. The real reason there are more impact craters on the far side of the Moon is that the near side has a thinner crust that allowed volcanoes to erupt about three billion years ago, and these large lava flows have covered the craters that were formed early in the Moon’s history. It is likely that each side of the Moon has received equal numbers of impacts, but the resurfacing by lava results in fewer craters being visible on the near side than the far side.

Question: I’ve recently been hearing a lot about something called global/solar dimming. Are we headed for another ice age or will some horrid catastrophe be caused by the Sun?

Global dimming or solar dimming usually refers to the reduction in sunlight reaching the Earth’s surface due to atmospheric aerosols (smog). During the middle of the twentieth century, the increase in industrial smog partly compensated for the greater greenhouse effect by adding carbon dioxide into the atmosphere, thereby slowing the global warming that would have been expected. One effect of the environmental movement of the last third of the twentieth century was to clean up the emission of smog, which had the unintended effect of accelerating global warming from the greenhouse effect. The other effect that you mention, a decrease in solar energy output during time of low solar activity, is much smaller. The Sun’s energy output (which we measure with satellites built for that purpose) varies by less than one percent, as you might expect since the number of sunspots or flares at the surface can’t influence the rate of nuclear fusion in the core of the Sun. The Sun’s brightness cannot drop enough to counteract current global warming due to the greenhouse effect. There will be no more ice ages if humans keep polluting the atmosphere with more carbon dioxide and methane and other greenhouse gases.

Question: A recent spate of furious eruptions on the Sun hurled a huge amount of heat toward Earth, the biggest dose our planet has received in seven years. The solar storm of March 8–10, 2012, deposited twenty-six billion kilowatt-hours of energy in the upper atmosphere, enough energy to power every home in New York City for two years. Would this intense heating have any effect on the Earth’s climate?

Your question is an excellent example of how solar activity is being hyped as if it were a major threat to Earth. The Sun is the source of almost all of our energy, and Earth intercepts almost 200 trillion kilowatts from the Sun continuously in the form of light and infrared (heat) radiation. Thus in the three days (seventy-two hours) of March 8 through 10, we received about 10¹⁶ kilowatt-hours of ordinary sunlight. Compare this with 26 billion kilowatt-hours from the solar eruption. The energy from the eruption is less than that from sunlight by a factor of nearly a million, negligible compared to regular sunlight. Solar outbursts make no contribution to heating the Earth or to global warming. In the report you read, don’t you think it would have been more realistic if they had noted this fact rather than talk about the solar storm in comparison to the energy needed to light New York City?

Remarkably, there are many new websites suggesting just that. For the moment, the cosmic conspiracy theorists are pushing aside the end of the world on 12/21/12 to make room for a more urgent threat from Comet Elenin, a faint long-period comet discovered in December. On Internet conspiracy sites, this comet is being blamed for the March 10 Japan earthquake and tsunami, and it is supposed to either hit the Earth or knock us off our axis in September. Many say it is not a comet at all but a black hole or brown dwarf star, or that it is a precursor of Nibiru, the 2012 death planet. Following are some questions and answers about Comet Elenin from the NASA “Ask an Astrobiologist” website.

Question: Please tell me about Comet Elenin; there does not seem to be much credible information. What is its size? I have heard it is twice the diameter of Earth. Some websites say that Elenin is 80,000 km in diameter including coma. Can you put something out there about what NASA knows about Comet Elenin, just to smooth things over before people get crazy? Some are claiming that the comet has disappeared or that there is a government plot to hide it.

Answer: I will be glad to tell you what I know about Comet Elenin. Most of the information comes from amateur astronomers, who have made hundreds of observations; see [http://www.aerith.net/comet/catalog/2010X1/2010X1.html]. Our knowledge of its orbit is constantly improving, with updates available from the Minor Planet Center [http://ubasti.cfa.harvard.edu/~cgi/ReturnPrepEph?d=c&o=CK10X010]. Elenin is estimated to have an orbital period of about 10,000 years.

There are many photos posted on the web, but be careful: the great majority of the images that come up in a Google search are not of Comet Elenin but of other comets or artist impressions (and some are fakes). For one good example see photos from Bernhard Hausler taken in early March [http://thewatchers.adorraeli.com/2011/03/11/new-approach-of-a-comet-with-an-asteroid-elenin/]. The comet is too faint to be seen or photographed with a small telescope; people who say the comet has disappeared or photos are being suppressed are lying.

To understand questions about the size of the comet, you must distinguish between the small solid nucleus of rock and ice and the large atmosphere (coma) and tail that develop as a comet approaches the Sun. The nucleus has not been measured but is probably about 4 km across. The coma was reported to be about 80,000 km across in early April, with a tail estimated at ten times that length. These are not unusual values for a comet, and both coma and tail are expected to grow as the comet approaches the Sun (it is still beyond the orbit of Mars at this writing on April 15). Some comets have developed atmospheres that can be more than a million kilometers across. But please remember that the atmosphere (coma and tail) is extremely tenuous—far less dense than the best vacuum that can be produced in the lab. This thin gas and dust can have no effect on the Earth or anything else.

Question: The photo of Comet Elenin that is usually shown is a fake image and it is actually comet 81P/Wild [http://www.suite101.com/view_image_articles.cfm/2683306]. NASA supposedly has limited info and no pictures. Oh, really? Not from the WISE satellite, designed specifically to look at low light objects (like dwarf stars) with high-sensitivity infrared? What about the South Pole telescope which has excellent IR detection capabilities? How about Hubble? Nothing! Is this really the dwarf star that would end all life on earth? Why doesn’t NASA ever say anything about it?

Answer: I am worried about the effects of the fear campaign about Comet Elenin that is being waged on the Internet. We all remember the Heaven's Gate cult that became so obsessed with Comet Hale-Bopp in 1997 that the members committed mass suicide [http://en.wikipedia.org/wiki/Heaven's_Gate_(religious_group)]. I don't want anything like that to happen again, so I will answer this and the following questions about Comet Elenin. Perhaps I can also promote some critical thinking about these claims.

(1) Yes, I know that this photo (which is not faked, just misidentified) is not of Comet Elenin. In fact, most of the images that come up when you Google "Elenin images" are not of Elenin. I don't know how this particular photo of Comet Wild became associated with Elenin, except that Elenin is so faint that perhaps some people wanted to substitute a picture of a brighter comet. (2) The WISE (Wide-field Infrared Survey Explorer) telescope ceased operations at about the time Elenin was discovered. This little comet, then in the asteroid belt, was probably too faint to be picked up by WISE. (3) I don't know much about the NSF telescopes at the South Pole, but in the five months since the discovery of Elenin, the South Pole has mostly been in continuous daylight. The South Pole would be one of the worst places to go to study this comet. (4) The Hubble Space Telescope will undoubtedly study Elenin when it is close to Earth, but I doubt that anyone has used it yet. Most astronomers will wait to study the comet when it is closer and brighter. (5) Do I need to remind my readers that a brown dwarf is billions (with a B) of times more massive than a comet? I suspect this brown dwarf rumor started when someone who is not familiar with the sky turned a small telescope on Jupiter with its four moons. (6) There is no reason that NASA (or the media) would be commenting on this small faint comet. The only people observing it now are amateur astronomers, who are tracking its orbit and its increasing brightness as it approaches the inner solar system. There is a lot of craziness circulating on the Internet, from people who either don't know much about astronomy or are intentionally making up stories to frighten gullible people. Shame on them!

Question: I read that a new calculation on Comet Elenin was posted on the NASA Buzzroom stating, "I've revised the 0.24 AUs down to 0.0004617 AUs on 16th Oct 2011" and that the Buzzroom forum was taken down due to that comment. I also read that "an inside source" from NASA is telling people that you guys are worried about the comet knocking the ISS out of orbit, which is why the space flights are being ended. I want to know if I am falling for the conspiracies trap. Where are these people getting their "updated calculations,” and who is this "inside source"? I truly dislike when people post these things in order to frighten others.

Answer: Those are indeed weird statements about Comet Elenin. Some of them come from an amateurish video that was posted anonymously on the NASA Buzzroom website in late February but is no longer available. There is no "inside source" from NASA. That is a standard technique of conspiracy websites: make up some story and then attribute it to a secret unnamed source so no one can check up on the claim. I don’t know details, but you are partly correct that the NASA Buzzroom website was taken down because of the video claiming that the comet orbit had changed. Also, other people had posted several offensive videos that had nothing to do with NASA or space. Because of these inappropriate postings, the entire Buzzroom website was taken down. I too get lots of offensive messages, but I don't post them. It is important that you be able to trust what you read on a government website.

NASA spaceflights are not ending, as anyone can tell by reading newspapers or looking at NASA websites. I am sure my readers realize that many people who are making comments on websites don’t know what they are talking about. Some think that the comet’s orbit will change suddenly and it may hit the Earth, whereas in reality it cannot come much closer than 100 times the distance to the Moon. Some question what mysterious force is pulling on it to bend its path; they seem to forget that this comet is in orbit around the Sun. Some think that we must know its mass to calculate its orbit; they have apparently forgotten the simple experiment credited to Galileo, who showed that objects of different mass behave the same way in response to gravity (for example, cannon balls of different mass fall at the same rate when dropped from a tower). One suggestion I saw was that perhaps Elenin is not a comet; the individual speculated that it might be a planet (perhaps Nibiru) or a brown dwarf or a massive black hole masquerading as a comet. Please think about what a comet is. By definition, a comet is a small object that sheds an extensive thin atmosphere of gas as it approaches the Sun. If it were massive (such as a planet, brown dwarf, or black hole), its gravity would hold on to the gas and it could not develop a coma or tail. Finally, there are the astrologers who seem to think that there are mysterious forces associated with alignments, but I am confident that no readers of this article would make such foolish errors.

Every month, as the Moon circles our planet in its elongated orbit, its distance from the Earth varies. At perigee (the Moon’s closest position to Earth), it is 14 percent closer than at apogee (the Moon’s farthest position from Earth) and therefore appears 14 percent larger. This change is too small to be noticed, unless you have some way to make a precise measurement of the Moon’s apparent size. In March 2011, the perigee happened within one hour of full phase.

The moment of full moon is also not easily apparent, and most people will call the Moon’s phase “full” over two or three days. So yes, the full moon was a little bit closer and brighter in March than usual, but if you missed it, it will be very nearly as close at the next full moon. The Moon’s perigee is nearly the same in every orbit, varying by less than 2 percent. Further, there is no evidence that the Moon is associated with anything on Earth except the tides. Neither earthquakes nor weather are correlated with the position or phases of the Moon. I received this question while attending the annual Lunar and Planetary Science Conference in Houston. About 1,500 scientists spent a week discussing fascinating new science, including spectacular high-resolution images from the NASA Lunar Orbiter, two space missions that have recently had close encounters with comets, and a flood of exciting new information about the planet Mars. Not one person mentioned 2012 doomsday or Nibiru, pole shifts, supermoons, or any of the other pseudoscience that circulates widely on the Internet. It is a jolt for me to shift attention from discussing exciting new scientific discoveries with my colleagues to answering questions I receive every day from people who fear imaginary threats like Nibiru or pole shifts or supermoons.

Question: There are reports and videos of two suns seen close together in the sky over China. What is in these videos; is it a second sun or planet? There was also a second sun seen in New Zealand at the time of the earthquake in Christchurch.

Answer: Let’s use common sense to analyze this question. (1) It is fascinating that people sit at their computer consoles and write to me about a second sun without stepping outside to see for themselves. If there really were a second sun in the sky close to the real sun, it would be equally visible everywhere to anyone who looked in the daytime. Since the image in the most popular video from China shows this “second sun” as being nearly as bright as the real sun, we would also be receiving nearly twice as much light and therefore be burning up. In other words, the idea of two suns in our sky is obviously untrue. (2) You mention that the second image could be a planet. Planets shine by reflected sunlight and are millions of times fainter than the sun. That is why it is so difficult for astronomers to photograph planets around distant stars. So we can easily reject that option. (3) Aren’t you concerned that the second sun images are being presented in videos, which are inherently low resolution? They are amateur videos at that with no accompanying documentation. These facts strongly suggest fakery. (4) A photo with two images of the same size side by side can be faked by shooting through double glass. The separation and relative brightness of the images depend on the spacing between the glass and the angle of the shot. (5) A few astronomers have suggested that this phenomenon could be produced under rare atmospheric conditions by a mirage. They may be right, and I would look seriously at these suggestions if I thought the double-sun photos were real. But scientists have been fooled in the past because they do not expect fake data. Professional magicians, who understand fakery, are often better skeptics than scientists.

Question: Everyone is freaking out about Comet Elenin, but no one has much real information. Has anyone determined its size and mass? If you don’t know its size and mass, how can you calculate its orbit? What are the chances that Elenin will either impact us or get close enough to cause a major catastrophe?

Answer: Comet C2012 X1 Elenin (to give its full name) is a small, long-period comet that takes about 10,000 years to complete one orbit around the Sun. Russian amateur astronomer Leonid Elenin discovered it with a robotic telescope in New Mexico on December 10, 2010. Astronomers have not measured its size because it is only a few kilometers across, and its solid nucleus is shrouded by the surrounding gas. The mass is too small to cause any change in the orbits of other objects, and so its mass is unlikely ever to be measured. However, we do not need to know either the size or the mass of a comet to calculate its orbit, as some readers may remember from their college physics or astronomy courses. It is precisely because Elenin is small and distant that journalists, and the public, have not shown much interest. Although there are still some uncertainties in its orbit, Elenin’s perihelion (when it will be closest to the Sun) is in early September 2011 at a distance from the Sun of forty to forty-five million miles. It will be closest to Earth on about October 16, 2011, at a distance of about twenty-one million miles, which is nearly a hundred times farther than the distance between the Earth and the Moon. It will probably be visible using binoculars during October.

Unfortunately, there is a rapidly growing list of conspiracy theory websites (apparently written by people who don’t know, or don’t care, what a comet is) making wild claims that Elenin will hit the Earth, disturb our orbit, cause changes to the tides, or interact with our magnetic field. Such claims are pure fiction. One of the worst examples is a video posted on March 1, 2011, claiming that the magnetic field of the comet would cause a large shift in the rotation axis of the Earth and produce mega-earthquakes on March 15, 2011. In reality, comets don’t have magnetic fields, and magnetic fields can’t change the rotation axis or cause earthquakes no matter how large they are. Unfortunately, the comet hysteria has grown since the tragic March 10 earthquake in Japan, which many pseudoscientific websites blame on this comet.

Question: Are you claiming that the March 10 earthquake in Japan and Comet Elenin are coincidences? Someone predicts an earthquake—basing it on Elenin orbit—and it happens. Isn’t it worth re-evaluating the prediction?

There is no connection between Comet Elenin and the March 10 earthquake. Scientific explanations depend on cause and effect. This comet can have no gravitational or electromagnetic effect on Earth. It is only a billionth of the mass of the Earth, and comets don’t have magnetic fields. Also remember that there was no large change (greater than 10 degrees was predicted) in the rotation axis of Earth, so there was no pole shift to trigger any earthquake. Equally important, earthquakes are not caused by external forces—not by gravity, electromagnetism, or pole shifts. Earthquakes are a product of the active geology associated with plate tectonics. The Japan earthquake was ordinary (although exceptionally large) and occurred on one of the most active subduction fault systems in the world. This is the same fault system that killed even more people in the great Yokohama quake and fire in 1923. Earth’s active geology caused this earthquake, not some poor little comet that is too faint to see without a telescope.

Question: NASA scientist Richard Hoover recently claimed to have found life in meteorites. Were these findings debunked or are they still inconclusive?

I have not yet met any astrobiologist who is convinced by Richard Hoover’s claims, which probably explains why you have not heard any updates on this story. One of the biggest problems is likely contamination by terrestrial microbes after the meteorite fell to Earth. It is also troubling that this result was not published in a real scientific journal but instead on an unreviewed online website. Over the past fifty years many scientists have investigated thousands of meteorites, and they have not seen anything that looks like life. Remember also that the meteorite parent objects are asteroids, not planets (except for the few meteorites we have from the Moon and Mars, which are in a special class). It would be unexpected to find life on a small, airless asteroid. This claim of fossil life is an example of an exceptional claim, and as Carl Sagan taught us, exceptional claims require exceptional evidence to be accepted. Hoover’s unreviewed paper is not exceptional evidence.

I am struck by the differences between this claim and the treatment given to the evidence for fossil life in a Mars meteorite reported in 1996. In that case, many scientists carefully reviewed the evidence, which was published in a prestigious scientific journal, yet it was presented at a NASA press conference as a tentative result with opportunity for critics to indicate their skepticism. Although the Mars claim is not generally accepted today, it stimulated a lot of excellent follow-up research. If Hoover wants to be taken seriously by the community of astrobiologists, he needs to publish his work in a real journal and respond to the criticisms from other scientists. That is the way science advances.

Storms of My Grandchildren: The Truth about the Coming Climate Catastrophe and Our Last Chance to Save Humanity
By James Hansen. Bloomsbury Press,
New York, 2009. ISBN: 978-160819-200-7.
301 pp. Hardcover, $25.

Science as a Contact Sport: Inside the Battle to Save Earth's Climate
By Stephen H. Schneider. National Geographic Society,
Washington, DC, 2009. ISBN: 978-1-4262-0540-8.
295 pp. Hardcover, $28.

Merchants of Doubt: How a Handful of Scientists Obscured the Truth on Issues from Tobacco Smoke to Global Warming
By Naomi Oreskes and Erik M. Conway. Bloomsbury Press, New York, 2010. ISBN: 978-1-59691-610-4.
355 pp. Hardcover, $27.

Anthropogenic (human-caused) global warming has become an emotional issue rife with political overtones. There is no dispute among climate scientists about the reality of global warming and the fact that it is primarily caused by accelerated burning of fossil fuels. However, there are nontechnical aspects of global warming that should interest readers of the Skeptical Inquirer: Specifically, how scientific evidence should be assessed in such a complex situation-and the way this evidence has been distorted and denied by a handful of scientists who are savvy about the politics of Washington and the media. As skeptics, we need to recognize the techniques used to distort and politicize the science. And as citizens, we should learn to recognize the similarities between these attacks on climate science and those used by creationists to undercut the fundamentals of biological science.

The three books I review here offer complementary frontline accounts of the Climate Wars. Both Jim Hansen and the late Steve Schneider have made important contributions to the development of climate science. Both started as modelers of the global atmosphere and ocean systems. Schneider led the climate team at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, and Hansen worked at (and now directs) the NASA Goddard Institute for Space Studies (GISS), located at Columbia University in New York City.

Schneider and Hansen, both members of the National Academy of Sciences (NAS), rose to prominence partly through their ability to explain complex climate issues to the press, to congressional committees, and ultimately to all of us. These two books of theirs are first-person accounts, geared toward the lay reader, that give about equal space to the scientific issues and to their own sometimes-controversial roles in them. The third book, Merchants of Doubt by Naomi Oreskes and Erik Conway, brings historical perspective to this issue. Oreskes and Conway compare the global-warming controversy to earlier campaigns to undercut the science that linked tobacco smoke to cancer, acid rain to smokestack emissions, and ozone depletion to the release of CFCs. This antiscience propaganda is a topic I will return to later in this review.

*...*

Jim Hansen is perhaps the most famous (or notorious, depending on your politics) climate scientist. This is ironic given Hansen's midwestern roots, his careful approach to science, and his conservative, shy personality. Although he began his career as a planetary scientist studying the atmosphere of Venus, he has been working since the 1970s on understanding Earth's climate, primarily as leader of a small group of climate modelers who use NASA's powerful computers to simulate the processes that influence atmosphere and climate. As his conviction that the Earth is in deadly peril due to the discharge of carbon dioxide into the atmosphere has grown, he has become an activist, first within the government as an advisor to the executive and legislative branches and more recently as an outspoken critic of “business as usual” practice and policies. Storms of My Grandchildren is his first book.

Hansen came to prominence in the hot summer of 1988, when he testified at a hearing before the Senate's Energy and Natural Resources Committee. He said that “Global warming is now large enough that we can ascribe with a high degree of confidence a cause-and-effect relationship to the greenhouse effect.” The next day the New York Times declared: “global warming is here.” There had been a growing consensus among climate scientists that greenhouse warming would take place, but Hansen was the first to state bluntly that it was already happening. People listened. By the end of the year, thirty-two climate-related bills had been introduced in Congress (none passed).

Hansen's book describes his adventures in the public spotlight, including interactions with Vice President Dick Cheney and President George W. Bush's Climate Task Force. We also hear about Hansen's experience in 1989 when he revealed, at a Senate hearing chaired by Al Gore, that the White House had altered his written testimony. In 2005 he again became the center of a controversy over censorship, as political appointees at NASA Headquarters tried to control even the basic temperature data that were being posted on the GISS website. When Andy Revkin of the New York Times exposed this heavy-handed attempt at political interference, the NASA Public Affairs officers backtracked and blamed the “misunderstanding” on a twenty-four-year-old intern who had faked his college degree and boasted that his job at NASA was “to make the President look good.”

Above all, Jim Hansen is a hard-working scientist. Much of his book concerns his research on the science behind global warming as published in peer-reviewed academic journals. Hansen explains the fundamentals of climate change in a way that is accessible to the nonscientist. He does not overwhelm us with the mathematics of the greenhouse effect or the results of complex computer models of atmospheric circulation. Instead, he appeals to the more basic idea of conservation of energy, using data about climate changes over the past several million years to derive fundamental truths about our current dilemma.

The climate system reacts to changes imposed upon it, which is called forcing. These changes include variations in the amount of energy received from the Sun, slow changes in the orbit and tilt of the Earth, and of course changes in the composition of the atmosphere and its clouds. Greenhouse forcing is positive, as several watts per square meter of additional energy are radiated from the atmosphere back to the surface. Forcing due to clouds and other aerosols that reflect part of the solar energy back to space is negative. The degree of climate change depends primarily on the balance between the positive forcing by the greenhouse effect and the negative forcing by aerosols. During much of the Industrial Revolution, the two types of forcing were roughly balanced because the same industries that burned coal to release carbon dioxide also poured smoke into the atmosphere. Since World War II, the output of greenhouse gases has greatly increased-not just carbon dioxide but also methane and industrial chemicals such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). At the same time, environmentalists succeeded in reducing the emission of smoke and other aerosols. Thus we now have strong positive forcing, leading to rapid global warming.

Hansen goes further, however. He notes that the current system is not in equilibrium; we are receiving more heat than is being radiated back into space. It is this imbalance that causes rising temperature and is starting to melt the polar ice caps. Much of this excess heat is also going into the ocean reservoir. It is this heat sink that assures us that global warming will continue for several decades, even if greenhouse forcing is held steady or decreased.

We can see from studies of past climate that temperature and atmospheric carbon dioxide are highly correlated. Most previous climate changes were much more gradual than those we are experiencing today, but their magnitude has been quite large (5 degrees Celsius in both directions). Hansen calculated the forcing implied from paleoclimate data to estimate the equilibrium temperatures we can expect as a function of the CO₂ content of the atmosphere. He concludes that with the present CO₂ concentration, we will raise sea level by several meters at minimum due to melting ice. If the CO₂ should double or triple, which is likely if much of the Earth's coal deposits are mined and burned, we will slip into a new regime in which most of the ice caps will melt, raising the ocean level at least fifty meters, which was the level when Earth's atmosphere contained this much CO₂ in the past.

These considerations, based in interpretation of paleoclimate data, convinced Hansen of the existence of a tipping point, when the changes we are producing become irreversible (on human timescales). He believes that we are dangerously close to that tipping point, and he is a strong supporter of efforts to limit the CO₂ content of the atmosphere to 350 ppm (which requires a return to the level of the late 1980s). As he frequently asserts, meeting this goal precludes us from burning most coal deposits or trying to extract oil from tar shale and other difficult-to-use forms of hydrocarbons.

Hansen's proposed moratorium on coal-fired generating plants directly confronts our requirement for “base load,” the steady electrical energy that cannot be supplied by solar or wind power. He describes a meeting with energy experts in Germany who said it was impossible to avoid building coal-fired generators because Germany is committed to a policy of phasing out nuclear energy. Hansen, reluctantly but firmly, is now an advocate for nuclear power, specifically for the new generation of clean-burning breeder reactors that can actually consume current radioactive waste. In reply to environmental organizations that oppose nuclear power, he notes that an estimated hundred thousand people per year die from exposure to coal, compared with at most a handful from our only serious nuclear accident at Chernobyl in 1986.

In his book we can see the gradual radicalization of Jim Hansen, self-described as a “placid, even comfortably stolid” atmospheric modeler and theorist. In 2006, Time magazine listed him among the 100 most influential people in the world. Hansen concludes this book by stressing that we face the most urgent fight of our lives and noting that civil resistance to new coal plants may be our only recourse.

*...*

Stephen H. Schneider, who died this summer (see “The Loss of Climate Scientist Stephen H. Schneider” in this issue), was a leading climate modeler much like Jim Hansen. But while Hansen is basically a reluctant public figure, Schneider epitomized the sophisticated scientist-politician; he was at home in the halls of Congress or on the front line of international climate negotiations. Winner of the MacArthur “genius” prize and co-recipient of the 2007 Nobel Peace Prize, he left NCAR for a senior professorship at Stanford University, providing him opportunities to mentor the next generation of atmospheric scientists. His wife, also a Stanford professor, is herself a world-class expert on the response of ecosystems to environmental change.

Schneider began to investigate the risk of climate change in the 1970s. As a graduate student he wrote a thesis on global cooling-when it appeared that negative climate forcing from smoke and other aerosols dominated over greenhouse heating, but the facts soon demonstrated otherwise. The danger of global warming was evident to many during the administration of Jimmy Carter, and a number of bipartisan government actions mandated increases in the efficiency of appliances and required substantial gains in automobile mileage performance. Schneider's first testimony before Congress was in 1979, a decade before Jim Hansen proclaimed the reality of global warming.

Two decades of bipartisan support for environmental science evaporated when Ronald Reagan became president. He and his appointees flatly denied the existence of either an energy problem or a threat of global warming. Famously, Reagan said that government is not the solution to our problems, it is the problem. The linked issues of energy policy and climate change have been sharply polarizing topics in the United States ever since.

In this book, Schneider recounts his personal story of the Climate Wars. The highlight-and the source of the book's title, Science as a Contact Sport-is his description of the Intergovernmental Panel on Climate Change (IPCC) and associated international events in which he participated, such as the 1992 Earth Summit in Rio de Janiero and the 1997 summit that approved the Kyoto Protocol. Along the way he also fought running battles with those who denied the reality of warming and used the media to distort and obfuscate climate science.

One of the common accusations of the climate contrarians is that the IPCC was a radical group in which alarmist scientists made unsupported statements about the dangers of global warming. It was thus with great interest that I read how this large international group, consisting of hundreds of scientists and thousands of reviewers and consultants, worked for three years on each of the three IPCC assessments. To hear the critics, one would think the radicals hijacked this process and then claimed consensus for their views. But as Schneider explains, the IPCC was forced to adopt the UN definition of “consensus,” which is “unanimity.” Every nation had to approve every word in the document, and even one veto could scuttle the entire process. Consequently, each statement of fact had to be documented, and every suggestion of policy approved by even the most conservative delegations.

Schneider's first-hand stories of these negotiations are fascinating. A handful of countries were the source of almost all of the objections: Saudi Arabia, the United States, China, and Russia-not coincidentally the biggest producers and consumers of hydrocarbons. One of the most frequent phrases in the IPCC notes is “Saudi Arabia, with the support of the United States, objected....” Schneider also reports that many of the members of the official U.S. delegation admitted privately that they agreed with the majority of scientists there but were under different orders from their bosses in Washington. He played a central role in finding compromises. At least once, he threatened to expose the obstructionist position of the U.S. delegation to the press and in congressional testimony scheduled for the next week. Schneider's description of the process explains why the IPCC assessments, far from being radical, 
have consistently underestimated future warming and its consequences.

Schneider's own contributions to the IPCC reports centered on providing meaningful estimates of uncertainties in both the anticipated climate changes and their likely effect on ecosystems and economies. Although the current state of the climate is measurable, all estimates of future trends are uncertain to varying degrees. Most of these predictions are based on computer models because, as Schneider often notes, you can't collect data about future conditions. To make such assessments useful to policymakers, the scientists must provide quantitative estimates of uncertainty, and Schneider insisted on a consistent vocabulary in the use of terms such as “likely,” “very likely,” and “high confidence level.”

Schneider also describes his outrage in 1996 when the second IPCC assessment report was released. In a long letter published in the Wall Street Journal, solid state physicist Frederick Seitz personally attacked the lead author of the report, accusing him of a blatant attempt to “remove hints of the skepticism with which many scientists regard claims that human activities are having a major impact on climate” and claimed that this was a falsification intended to deceive the public and policymakers. The accusation was flatly untrue. But what amazed the scientists was that Seitz was a past president of the NAS. Why was Seitz, who was not a climate scientist, launching this personal attack-which was quickly echoed by Fred Singer, another senior scientist, in a Wall Street Journal article, “Coverup in the Greenhouse”?

*...*

The third book in my trilogy deals with the role of contrarians like Fred Seitz and Fred Singer and their sophisticated campaign to undercut the science of global warming. They had previously also disputed the reality of ozone depletion and the links between cigarette smoking and cancer. These individuals and organizations are the “merchants of doubt” referred to in the title of the book by historians Naomi Oreskes and Erik Conway.

Merchants of Doubt places the attacks on climate science and the IPCC in a broader context. Seitz, Singer, and a handful of other scientists have waged a thirty-year campaign against a wide range of environmental issues. They followed what is often called the “Tobacco Strategy.” In the 1950s and '60s, evidence rapidly grew that cigarette smoking causes lung cancer and a variety of other diseases. The tobacco industry could not prove this claim wrong, but they could and did try to undercut the science behind it. Internal tobacco memos state that “doubt is our product.” Big Tobacco used science to fight science by funding a few pliable academic researchers and setting up nonprofit foundations and organizations that released “scientific reports” and engaged in other forms of “education.” Although they lost this fight, their efforts delayed effective government action by more than two decades and indirectly led to the premature deaths of millions of smokers, who were happy to believe that the link between smoking and cancer had not been proven.

When evidence began to accumulate in the 1990s that there was also a link between secondhand smoke and disease, the same techniques were used by the tobacco companies, this time with the support of a few famous scientists such as Fred Seitz (former president not only of the NAS but of Rockefeller University), Fred Singer (first director of the National Weather Satellites Service), Bill Nierenberg (former director of the Scripps Institution of Oceanography), and Robert Jastrow (founding director of the NASA GISS).

These four retired physicists had worked on Cold War defense projects and held senior advisory positions in the Reagan administration. They first gained national prominence for their outspoken support of the Strategic Defense Initiative (SDI, commonly called “Star Wars”). Jastrow, Seitz, and Nierenberg founded the George C. Marshall Institute, a tax-exempt educational organization promoting “science for public policy” with a goal to “raise the level of scientific literacy ... with an impact on national security.” They were strongly anti-communist, opposed to co-existence or detente with the Soviet Union, and in favor of much larger defense budgets. Many of their education efforts were directed at journalists and congressional staff. The Marshall Institute joined other think tanks with similar political goals, such as the Heritage Foundation, the Hoover Institution, the Hudson Institute, the Competitive Enterprise Institute, and the Cato Institute.

Oreskes and Conway describe several of their campaigns in detail, beginning with their assertions that a “Star Wars” missile-defense system was both feasible and affordable. As physicists who had worked on defense programs, their opinions had considerable weight. At the same time, however, they went well beyond their areas of expertise to question the environmental harm of acid rain and oppose regulations to reduce smokestack emissions. This campaign, led by Fred Singer (affiliated with the Heritage Foundation and a White House insider), placed them in direct opposition to the NAS and 
the Environmental Protection Agency. They were aligned with the Reagan administration, however. “We don't know what is causing it” became the official position of the U.S. government, and action was not taken to mitigate acid rain until 1990, under the first Bush administration.

Meanwhile, there was a growing scientific concern over depletion of ozone in the stratosphere. Ozone blocks short-wave ultraviolet sunlight that would otherwise be a risk for all life on land. The primary culprit was CFCs, which were used as refrigerants and spray-can propellants and to clean electronic components. Billions of pounds of CFCs were being manufactured every year. In their defense, the chemical companies, through their trade organizations, began to follow the tobacco strategy. They dispensed millions of dollars in research grants and established several organizations for public relations purposes, such as the Aerosol Education Bureau. The industry promoted the idea that volcanic eruptions, not CFCs, were destroying stratospheric ozone.

In 1985, public interest and concern were stimulated by the discovery of the Antarctic ozone hole. Suddenly the invisible chemical changes in the stratosphere were made visible by satellite images of the Antarctic. A counternarrative was soon developed, led by Fred Singer. Writing in the Wall Street Journal, Singer criticized the “ozone scare” and asserted that there is no proof of ozone depletion or of a cause-and-effect relationship with CFCs. His thesis was that the science is uncertain, replacing CFCs will be difficult and expensive, and the scientific community is corrupt and motivated by self-interest and political ideology-the same arguments later used by global warming deniers.

As late as 1995, after the Montreal Protocol had banned most manufacture of CFCs, Singer testified before Congress that the scientific concern about ozone depletion was simply “wrong.” He attacked the Swedish Academy of Sciences when the Nobel Prize in Chemistry was given for analysis of stratospheric ozone chemistry. Singer described his motivation in 1989 this way: “There are probably those with hidden agendas of their own-not just to save the environment but to change our economic system. Some are socialists, some are technology hating Luddites; most have a great desire to regulate on as large a scale as possible.” In 1991 he wrote that the real agenda of environmentalists was to destroy capitalism and replace it with some sort of worldwide utopian socialism-or perhaps communism. (All taken from Merchants of Doubt, which has extensive documentation.)

With this background, it is easier to understand Fred Seitz's 1996 attack on the second IPCC assessment. Actually, the Marshall Institute had begun attacking climate science in 1989, the year the Berlin Wall fell. The Institute published a booklet titled “Global Warming: What Does the Science Tell Us?” by Jastrow, Seitz, and Nierenberg, blaming whatever global warming might be happening on the Sun. Nierenberg himself briefed the report to the White House. The contrarians founded a faux-scientific journal called the World Climate Review, partly funded by fossil-fuel interests. They used the right-wing Washington Times and the Wall Street Journal, knowing that these papers would not publish rebuttals from the climate scientists.

Even as the scientific case for anthropogenic global warming became more secure, Senator James Inhofe (then-chair of the Senate Committee on Environment and Public Works) called global warming “the greatest hoax ever perpetuated on the American people.” In 1997, the U.S. Senate passed a resolution blocking adoption of the Kyoto Protocol by a vote of 97–0. In Washington, politics, money, and ideology were in ascendancy over science. The situation became even worse under George W. Bush, with the spectacle of science-fiction writer Michael Crichton appearing as an expert witness before Congress and lecturing at the White House on global warming.

The preceding paragraphs only hint at the detailed history of the science-contrarian movement described by Oreskes and Conway. It is frankly difficult for a scientist to believe that other scientists would consciously misrepresent the scientific facts, but the pattern is inescapable, as Seitz and Singer and a handful of others have taken the same denialist position on one issue after another: tobacco and cancer, acid rain, ozone depletion, DDT, secondhand smoke, and now global warming. As documented in this book, these contrarians are fighting science because of their political conviction that government regulations are in themselves evil. They believe that environmentalism leads to regulation, which will inevitably lead to the loss of our freedom. They call environmentalists watermelons: green on the outside but red on the inside. Having fought for freedom all their lives against the evils of communism, they are now fighting just as hard to protect our liberty from the evils of government regulation. That mission apparently trumps their devotion to science.

*...*

These three books were written before the “climategate” accusations of late 2009, which were used effectively to smear the IPCC specifically and climate scientists generally. Most of the media went along with this sophisticated public-relations campaign, describing this as the greatest crisis faced by science in a generation. Some media later printed retractions and even apologized when impartial investigations showed that the accusations were false. But this episode demonstrated the ability of the contrarians to influence public opinion. I imagine that Oreskes and Conway could add this episode to their long list of public-relations battles, but it did nothing to undercut the reality of global warming or the serious implications it has for our future.

There are many parallels between this campaign against climate science and the widespread efforts to deny biological evolution and block its inclusion in science classes. Both climate denialism and evolution denialism are efforts to fight real science with pseudoscience-these contrarians publish fake science in non-refereed journals, found NGOs, and use sophisticated marketing in the halls of power from Washington to Austin. Apparently even good scientists can be 
seduced by a strong ideological commitment, just as the creationists are much more interested in saving our souls than in addressing the science of biology. Like creationists, the climate contrarians are “merchants of doubt,” using pseudoscience to undercut real science and create a wedge for their nonscientific beliefs.

I recommend all three of these books. They are well written, timely, and provocative. Hansen emphasizes the science and introduces some novel ways to assess future warming without invoking complex computer models. Schneider paints a fascinating picture of the struggle to develop a consensus on global warming, especially through the UN IPCC assessments. Oreskes and Conway place the climate controversy in perspective as the most recent example of how ideology and politics have been used (with considerable success) to attack science. Their message is one that skeptics everywhere will want to read and ponder. n

Note

Of the four leading climate denialists discussed here, only Fred Singer is still going strong. Bill Nierenberg died in 2000, and both Frederick Seitz and Robert Jastrow died in 2010.

I thank several colleagues for helping to educate me on climate science or for offering constructive criticism of this manuscript: Mark Boslough, Clark Chapman, Bob Chatfield, Erik Conway, John Mashey, Naomi Oreskes, Phillip Russell, and Eugenie Scott.

Question: If there is no problem coming in 2012 then why is the government of the United States in such a rush to build so many underground bunkers? On Jesse Ventura’s Conspiracy Theory on TV they showed the inside of bunkers and they also showed U.S. security forces. They stated they’d do what ever it takes to keep the bunkers secure in 2012.

There are no bunkers being built in anticipation of 2012, except possibly by some individuals who don’t realize that doomsday 2012 is a hoax. I expect that Jesse Ventura knows 2012 is just a hoax, but he is in show business. His show Conspiracy Theory is entertainment, not journalism. The name gives this away because “conspiracy theory” is a pejorative term that refers to any fringe idea that explains a historical or current event as the result of a secret plot by conspirators. Watch Ventura’s episode on 2012 carefully and you will see that he shows no actual government bunkers and interviews no one who claims to have seen them. It is all innuendo and speculation, with films of people (in the Denver airport for example) who are obviously confused by this guy asking weird questions and wanting access to “bunkers.” He does show one example where private developers are constructing homes inside an abandoned missile silo. Notice that Ventura did not contact the people who are planning to move into one of these underground homes to ask them why they are moving there. This show is for entertainment, so it is better to leave questions unresolved.

In reality, there are bombproof civil defense bunkers all around the U.S. that were built during the cold war, especially during the Eisenhower administration. These include the famous Cheyenne Mountain operations center of the Strategic Air Command and the large complex at Greenbrier, West Virginia, which was once the designated site for senior government officials in case of atomic attack on Washington. (There are good Wikipedia articles on both). Civil defense shelters are an old story, and they have nothing to do with 2012.

Question: Should we be worried about the gradual warming of the Sun?

No, but you should be worried about the rapid warming of the Earth that we are experiencing today. The Sun influences climate in two ways. First, there is the periodic variation in solar energy output related to the eleven-year sunspot (or solar activity) cycle. The temperature changes due to this variation have been carefully monitored from space for several decades, and they are very small—almost too small to be detected in global temperature measurements.

Second, there is the gradual warming that has taken place since the Sun was formed more than four billion years ago, which will continue for billions of years into the future. As it ages, the Sun brightens by about 7 percent per billion years, which is negligible on any timescale of less than tens of millions of years, although it will make Earth uninhabitable some two to three billion years in the future.

Our current climate crisis is not related to the Sun. It is the direct response of the Earth to the added carbon dioxide and methane and other greenhouse gases in the atmosphere. More than century ago, Nobel Prize-winning chemist Svante Arrhenius first predicted that the release of carbon dioxide from burning fossil fuels would cause additional greenhouse warming. Now we are seeing this as a dramatic climate change that places our entire civilization at risk.

There are interesting parallels between the campaign against climate science and the widespread efforts to deny biological evolution and block its inclusion in science classes. Both climate denialism and evolution denialism use pseudoscience to fight real science. They include founding of nonprofit institutes, publication of fake science in non-refereed journals, and sophisticated marketing in the halls of power. Apparently some scientists can be seduced by a strong ideological commitment to fight government environmental regulations, just as some creationists are much more interested in saving our souls (and theirs) than in addressing the science of biology. Like creationists, many of the climate contrarians are “merchants of doubt,” using pseudoscience to undercut real science and create a wedge for their nonscientific beliefs.

Question: If the world does not end in 2012, then when and how do you predict that it will end? Will it be a painful death?

The “end of the world” is a silly idea. For the first five years that I answered questions sent to “Ask an Astrobiologist,” I never encountered any questions about the end of the world. Now I receive at least one per week. My guess is that the people who ask these questions are not interested in the fact that the Sun will become a red giant and consume the Earth in about four billion years; no one expects the human race or human civilization to last for billions of years. These question askers seem to be fearful of something that will happen in their lifetimes or perhaps the lifetimes of their great-great grandchildren.

I promise you that there is nothing that could destroy the Earth. The only two possible global threats in the next few centuries or millennia are a collision with a large comet—which is extremely unlikely but possible—and the melting of the polar ice caps due to global warming, which is inevitable if we don’t dramatically alter our consumption of fossil fuels. Either global warming or a comet collision could lead to a mass extinction and perhaps the destruction of our civilization, but neither will be the end of the world or even of humanity. The problem with the question about the end of the world is that it raises needless fears and distracts us from dealing with the real problems we face.

Question: Lately there have been reports that the next solar maximum, or “solar tsunami,” is expected to be devastating to mankind. Meanwhile, it seems that the recent solar flare ejections are already starting to cause panic among some people, some of whom are saying NASA now expects the solar maximum to bring death. I’ve just watched a Discovery Channel show about solar storms. Looking at all that devastation still gives me goosebumps.

The Sun is not our enemy and will not hurt us. NASA has never suggested that the solar maximum could bring death. A lot of shows on the Discovery Channel exaggerate dangers; apparently such shows attract viewers and advertisers. The simple facts are that the Sun has an eleven-year activity cycle, but we cannot accurately predict individual events such as flares and coronal mass ejections (CMEs). In the case of the largest CMEs, it is possible to damage transformers in electrical grids, and NASA is currently planning to provide accurate warnings to electrical utilities so they can isolate any parts that are likely to be directly affected by solar particles.

Solar scientists expect that the next solar maximum will be in the spring of 2013 and will be unusually weak, but the Sun may have some surprises for us. However, there is nothing about this solar maximum that can hurt us or threaten devastation to mankind. There will be many solar flares between now and 2015, but there are no specific predictions. I think the use of such terms as “solar tsunami” or “solar storm” to describe solar activity is unfortunate. It makes people think of destructive tidal waves or storms on Earth—but the Sun is 150 million kilometers away! There is an especially confusing Fox News interview with cosmologist Michio Kaku that is popular on YouTube. Kaku states that at solar maximum the Sun’s magnetic field suddenly flips and a giant shock wave of radiation heads for Earth, which he warns could disable our civilization. This is not the way the solar cycle works, and it raises greatly exaggerated fears.

The current fear of the solar maximum reminds me of some of the stories that circulated around Y2K. In both cases there were legitimate concerns: to protect older computer systems from the “Y2K bug” and now to protect satellites from solar outbursts. But the dangers to individual people are hugely exaggerated. Assuming you were not killed or seriously hurt by the previous solar maxima in 2001 and 1990, you have nothing to fear from this one.

Question: I really want to believe Nibiru is not real. But why are people still talking about it on YouTube? Do you think the topic will die down? And if it grows, what can we do about it?

This 2012 hoax has been going strong for the past three years, and it shows no sign of dying down. Maybe a few people who post these crazy lies on the Internet and YouTube really believe it, but I suspect most of them are in it for the money. For the truth, I recommend the websites http://2012hoax.org and http://astrobiology.nasa.gov/ask-an-astrobiologist/intro/nibiru-and-doomsday-2012-questions-and-answers. I just returned from a meeting of professional science educators where we discussed at length how to respond to this doomsday hoax. Most of the educators I talked with plan to do more in their communities to inform people, especially to calm the fears that many children have about the end of the world. We are all concerned about the children who write that they are considering suicide before the world ends in 2012 and the mothers who have written that they intend to kill their children and themselves before the destruction begins.

Question: For many years, I have considered Planet X to be a myth. But in the past few weeks I have noticed a very bright “star” to the west, and it is very active. If it is Venus, it is much larger than usual. What do you think it is?

It is Venus. Others have written to me also suggesting it is Nibiru or Planet X, and similar claims were made last year about Jupiter. You can easily look up planet positions on the Internet, for example at www.skyandtelescope.com. I should add that while Venus is very bright, it is not large. All planets are so small that without a telescope they are seen as unresolved point sources. If Venus really looks large to you, you probably need new glasses or contacts. Within the next few months Jupiter (in the east) will displace Venus (in the west) as the brightest planet in the evening sky, and I anticipate that some people will also mistake it for Planet X. I wonder, though, how people can think they have discovered a brilliant planet that 100,000 astronomers have all missed.

Question: I have been told that in our night skies there were two moons visible on August 27, 2010, when Mars approached Earth at the nearest point in their orbits. Is this true?

This is a zombie question: dead a long time but still sure to pop up every summer. Everything you heard is wrong. This past summer Mars was very far from Earth, and it could not be seen even with a telescope. This silly rumor first surfaced in August 2003, when Mars was indeed very close to the Earth (about 55 million kilometers), but the claim about it being as large as the Moon is pure fiction. This is an example of how so much on the Internet is wrong and how difficult it is to get rid of such errors.

David Morrison is a planetary scientist and director of the NASA Lunar Science Institute. He is a Committee for Skeptical Inquiry fellow and a recipient of the American Astronomical Society’s Carl Sagan Award for science popularization. E-mail: david.morrison@nasa.gov. These (slightly edited) questions from the public were received through NASA’s Ask an Astrobiologist website (http://astrobiology.nasa.gov/ask-an-astrobiologist/0).

The rise and fall of the theory that cosmic catastrophes altered human prehistory in North America.

Ever since the Alvarez (1980) hypothesis that the end-Cretaceous (Cretaceous-Tertiary or KT) mass extinction was the result of a cosmic impact sixty-five million years ago, the idea of killer asteroids or comets has been frequently discussed. The stunning confirmation of the KT impact initiated a revolution in our thinking about possible external events and their effects on biological evolution. David Raup of the University of Chicago famously proposed that perhaps all major mass extinctions were impact induced. He even published a "kill curve," suggesting that lesser extinctions might be the result of smaller impacts. Unfortunately for those of us who sought a general explanation for mass extinctions, these broader suggestions have not been verified. It seems increasingly likely that cosmic impacts are only one of several catastrophic events that have produced mass extinctions. Still, the discovery that an impact sixty-five million years ago led to the extinction of the dinosaurs remains one of the iconic ideas of late twentieth century science.

The most dramatic recent hypothesis linking extinctions with impacts was proposed in 2007 by a team of twenty-six scientists, led by nuclear chemist Richard Firestone of Lawrence Berkeley National Laboratory, with independent geophysicist Allen West; geologist James Kennett of the University of California, Santa Barbara (a member of the National Academy of Sciences); and archaeologists Douglas Kennett and Jon Erlandson of the University of Oregon. In a widely reported presentation at a joint assembly of the American Geophysical Union (AGU) in Acapulco, Mexico—followed a few months later by a paper in the Proceedings of the National Academy of Sciences (PNAS)—these scientists proposed a cosmic origin for a geologically recent event, the extinction of many large mammals (megafauna) in North America approximately 13,000 years ago. The events they linked were the presence of a dark soil layer that coincided with the extinction of megafauna (including the mammoth and mastodon), the end of the Clovis culture (identified by its large and well-made spear points), and the start of the Younger Dryas (YD) cool period (a millennium pause in the general warming at the end of the last ice age). The in situ bones of extinct megafauna, along with Clovis stone tools, occur below this black mat but not within or above it. At this boundary the team reported finding enriched levels of iridium and other signatures of extraterrestrial material.

In a sweeping conclusion reminiscent of the Alvarez hypothesis, Firestone and his colleagues postulated that these events were tied to one or more cosmic impacts over North America, releasing energy they estimated at about ten million megatons (equivalent to an impacting comet four kilometers in diameter). They suggested that an airburst and/or surface impact by a dense swarm of carbonaceous asteroids or comets set vast areas of the North American continent on fire. This swarm would have exploded above or even into the Laurentide Ice Sheet north of the Great Lakes. Such an airburst would have been a million times larger than the Tunguska impact event of 1908.

Scientific Reactions

While archaeologists pondered the reality of this sharp boundary layer and the new evidence of extraterrestrial materials, a few astronomers and impact experts immediately questioned this scenario. They noted that there was no mechanism to hold such a dense swarm of impactors together in space. To the suggestion that a large comet had broken up just before hitting Earth, they replied that this lacked a physical mechanism. If the comet had shattered when it encountered the atmosphere at an altitude of about one hundred kilometers, the lateral dispersion would be at most tens of kilometers, hardly enough to distribute the effects across North America. An alternate suggestion was that this event was analogous to the 1992 tidal break-up of comet Shoemaker-Levy 9, which resulted in the separate impact of about twenty-three fragments on Jupiter two years later. However, these comet fragments were spread over more than a million kilometers in space, and the impacts were distributed over all longitudes on Jupiter. While it is true that some comets have been seen to spontaneously disintegrate in space, the chances of this happening just before an impact with Earth is negligible—something that might have happened at most once in the past four billion years. There was apparently no way to get a swarm of impactors to target North America alone.

One of Firestone and his colleagues' suggestions that troubled geologists and impact experts was that the same event (or a similar one) might have been responsible for the Carolina Bays geologic formation. The Carolina Bays are several hundred thousand shallow, elliptical depressions of disputed origin along the U.S. eastern seaboard. Firestone suggested that each of these more than 100,000 features was the result of a cosmic impact. Since the well-known Tunguska airburst in Siberia in 1908 did not form a crater, the implication is that these were made by larger objects that reached the ground. But calculation of average impact frequency suggested that only about one super-Tunguska could be expected to hit Earth in the past 13,000 years. The chances of two such extremely unlikely swarm impacts happening within the past few thousand years is worse than negligible.

A warning of the problems with this hypothesis should have been apparent to anyone who read 2006's The Cycle of Cosmic Catastrophes: Flood, Fire, and Famine in the History of Civilization by Richard Firestone and Allen West, with writer and publicist Simon Warwick-Smith. This trade book, which appeared a year before Firestone's AGU presentation, described the YD impact hypothesis as part of a much larger cycle of cosmic events. This book develops Firestone's 2001 suggestion that a cosmic ray catastrophe, probably caused by a supernova, occurred in northeastern North America in the late Pleistocene. He concluded that massive thermal neutron irradiation radically altered the radioactivity of terrestrial materials and "probably figured in the mass extinction of Ice Age fauna." In The Cycle of Cosmic Catastrophes, Firestone links the YD impact to this postulated nearby supernova, which he asserted took place 41,000 years ago and initially devastated most life in Asia. Then 34,000 years ago the shock wave from this supernova initiated another wave of intense cosmic bombardment of Earth. The only evidence for this event is the remarkable claim that mastodon tusks from about that time are pitted with cosmic dust, suggesting that these animals received the direct blast of supernova material striking Earth (unstopped, apparently, by our atmosphere).

In The Cycle of Cosmic Catastrophes, the debris cloud from the supernova is supposed to have reached Earth about 13,000 years ago. The YD impact was one manifestation of this blast wave, bathing the planet in radioactivity and destabilizing the magnetic field. In this book the authors suggest that the Carolina Bays were created by secondary impacts of ejecta from the main hit in the North American ice sheet. To produce this much ejecta, the hit must have been among the most catastrophic events in Earth's history. They suggested that the YD impact excavated Hudson Bay, making it larger than the KT impact of sixty-five million years ago, which is estimated to be a once-in-one-hundred-million-years event. Yet supposedly this huge hit did not produce a worldwide mass extinction but influenced only the megafauna of North America. This entire scenario is inconsistent with what astronomers know about supernovas, which Phil Plait summarized in his recent book Death from the Skies. It raises serious questions about the reliability of the PNAS paper that Firestone and West, with two dozen additional authors, published a year later.

New Data and Continued Controversy

In January 2009, Doug Kennett published a paper in Science asserting that nanodiamonds provide the strongest evidence for the impact hypothesis, with multiple airbursts and impacts at the onset of the YD cooling. He argued that these nanodiamonds were produced in the moderate shocks associated with comet airbursts. By this time, earlier claims about iridium enrichment and other possible impact markers had been withdrawn. The usual geological evidence of large crater-forming impacts such as the KT, namely shocked quartz, had never been reported at the YD boundary sites. Now the nature and origin of nanodiamonds became the primary issue.

There were a variety of claims and counterclaims concerning the nanodiamonds. Were they produced in the impact, or were they primordial material trapped in the comet when it formed billions of years earlier? Most impact experts agree that nanodiamonds were unlikely to have been formed in the impact. In fact, Mark Boslough of Sandia National Laboratories calculated that the high temperatures and pressures in a large impact would likely destroy existing nanodiamonds. Some note that nanodiamonds are actually ubiquitous on Earth and can even be formed in fires. One scientist joked that perhaps the nanodiamonds were concentrated at human habitation sites where hunters were roasting the meat from mammoths and mastodons. The history of these claims and counterclaims is well documented in articles by Science journalist Richard Kerr published in 2007, 2008, and 2009.

At a meeting of the Geological Society of America (GSA) in October 2009, several presentations argued strongly against the YD impact from a variety of perspectives (see GSA summary in references). One paper claimed that the black mats at the YD boundary were not charcoal from widespread fires but rather peat-rich dark soils formed during a wet period. Another speaker noted that there was no archeological evidence for a sudden decline in the human population of North America at the YD. While one speaks of the end of the Clovis culture, this only means that the style of stone tools changed. We don't know why, although one possibility is a shift to hunting smaller animals. Other scientific teams reported that their efforts in the field to find nanodiamonds or other impact markers at the YD boundary layer were unsuccessful.

Some impact proponents who were not present at the GSA meeting wrote blogs and circulated e-mails accusing these scientists of sloppy fieldwork. They asserted that the boundary layer was very thin and rather spotty in distribution, requiring care to find it—care they implied had not been exercised by their critics. The GSA session resulted in the undercutting of the credibility of the original PNAS and Science papers, but since the two sides did not confront each other directly, nothing was settled.

The American Geophysical Union Symposium

Given the conflicting interpretations concerning a possible YD impact catastrophe, many scientists thought a debate between proponents and critics might help clear the air. The YD impact hypothesis had been discussed for more than two years without any common ground emerging. Indeed, the original team of twenty-six scientists was itself fragmenting, with only Richard Firestone and Allen West still strongly advocating the original multi-comet impact scenario. Mark Boslough of Sandia worked with Allen West to organize a symposium at the 2009 fall meeting of the AGU, with speakers from both sides. While no one expected that public discussion would lead to reconciliation, the organizers hoped this symposium would at least focus on the main issues.

The December 2009 AGU session topic was "Younger Dryas Boundary: Extraterrestrial Impact or Not?" Ten speakers were squeezed into a single two-hour session, including Allen West, impact specialist Peter Schultz of Brown University, and former NASA geoscientist Ted Bunch from among the original PNAS authors. Firestone chose not to attend. There was standing room only at the session, and several hundred others were turned away at the door.

The star of the event was Wally Broecker of the Department of Earth and Environmental Sciences at Columbia University. Broecker is one of the most respected environmental scientists in the world. Credited with first describing the ocean current conveyer belt and inventing the term "global warming," his honors include membership in the National Academy of Sciences and award of the Presidential Medal of Science. His presentation was sober and low key, but he made it clear that he was unconvinced by the evidence for an impact or any catastrophic change at the YD boundary. But rather than condemning the hypothesis, he stated simply that the decline in the North American megafauna could be understood as a result of climate change and overhunting—the conventional explanation. Broecker said, "We do not need the impact hypothesis."

Most of the speakers who followed Broecker restated positions that were already on the record. West and his colleagues repeated their evidence of extraterrestrial markers in the black mat at the YD boundary, with emphasis on the presence of nanodiamonds. They suggested several possible impact scenarios, such as oblique impact on the ice sheet, but admitted that there were many uncertainties. Several critics reiterated that the proposed impact is highly unlikely statistically and that an airburst as large as proposed is inconsistent with our understanding of comets and the impact process.

The most interesting new results were presented by Jacquelyn Gill, a graduate student in the Department of Geography at the University of Wisconsin–Madison. She has been studying lake sediments that contain spores of sporomiella (a fungus that occurs in herbivore dung) in the time range around the YD. This fungus is related to the total mass of herbivores and can be used as a proxy for the megafauna population. Her data show a gradual decline, beginning well before the YD marker and extending beyond the end of the Younger Dryas cool period. Indeed, in some isolated locations mammoths and mastodons did not go extinct until much later: there were dwarf wooly mammoths on Wrangle Island in Alaska until about four thousand years ago. Some large North American mammals did not go extinct at all, including the bison, the moose, and the grizzly bear. Gill's results seem consistent with the worldwide evidence that rapid declines in large mammal population accompanied the arrival of early human hunters, presumably as a consequence of overhunting.

Unfortunately, the overcrowded session ran late, and there was no time for discussion or questions. Even when their conclusions were challenged, most of the scientists in the audience chose not to respond. The result was a lost opportunity for real debate. Perhaps not surprisingly, the AGU session received very little press attention. Indeed, following the AGU and GSA meetings, the YD impact hypothesis seems to have retreated into the obscurity of a few e-mail list-serves and blogs, such as "The Cosmic Tusk" where George Howard (one of the original PNAS authors) is presiding over a variety of catastrophist interpretations of Holocene history.

Conclusions

It is instructive to compare the trajectories of the YD and KT impact hypotheses, as there are close parallels. Both research teams were led by nuclear scientists (Luis Alvarez and Richard Firestone) from the University of California, Berkeley. Both challenged the orthodoxy of mass extinctions. Both postulated an environmental catastrophe triggered by a large cosmic impact. Both were published initially in prestigious journals (Science and PNAS). They each presented a grand synthesis, not only identifying evidence of extraterrestrial materials at the extinction boundary but also proposing a broad impact scenario to explain a wide variety of previously unrelated data. And both ideas were initially resisted by the "old guard" of paleontologists and archaeologists.

While each hypothesis encountered initial resistance, the KT impact theory also gained enthusiastic support (see popular accounts by Walter Alvarez and James Powell). The first confirming paper was published within weeks, and soon multiple impact markers had been identified at a number of additional exposures of the KT boundary. Astronomers and geologists praised the paper and provided context by estimating the impact rate for ten-kilometer comets and asteroids. Atmospheric scientists such as Brian Toon and Kevin Zahnle of NASA Ames Research Center calculated the dispersion and lifetime of dust ejected into the stratosphere by the impact. Paleontologists like Peter Ward (University of Washington)—who initially argued for a gradual decline of populations—gathered new field data and used modern statistics to support an abrupt extinction at the KT boundary. Within three years the first of a series of Snowbird Conferences was held, bringing together top scientists to discuss the role of cosmic impacts on the evolution of life. The idea of an impact extinction gained early and continuing currency in the press.

In contrast, efforts by other scientists to confirm the presence of impact markers at the YD boundary have so far been unsuccessful. Astronomers, rather then welcoming the impact idea, have raised serious objections to the proposal by Firestone and colleagues. New data on megafauna extinction, such as the work of Gill, point to a gradual decline. Archaeologists emphasize that changing styles in stone tools do not demonstrate a sudden shift in human populations at the start of the YD but merely a change in technology or hunting style. In the aftermath of the 2009 GSA and AGU meetings, the press seems to have lost interest, and continuing support for the YD impact comes mostly from blogs by catastrophists who have long advocated cosmic intervention in human history.

Even without considering the technical issues at stake, there are two clues that something is amiss with the YD impact hypothesis. First is the 2006 book The Cycle of Cosmic Catastrophes, which formulates the YD hypothesis within the context of catastrophist pseudoscience. If more scientists and science journalists had been aware of this earlier publication when the YD hypothesis was first published in PNAS, it might never have gained traction. Second is the absence of confirming or supporting papers by scientists who were not members of the original team. A good hypothesis naturally accretes confirmation and gets better with time, as did the Alvarez KT impact hypothesis. Firestone's work has not done so.

It seems clear that the YD impact proponents were trying to follow in the footsteps of the Alvarez team, discovering evidence of a sudden extinction event and linking this to an extraterrestrial impact. However, the story isn't working out that way, and the impact they propose seems to be virtually impossible. One parallel that troubles me, however, is that the reaction of the traditionalists—scientists who say that the megafauna were in decline anyway and "we don't need an impact"—rather closely echoes the reaction of many old-guard scientists to the KT impact hypothesis. There also may be philosophical and political overtones that influence the reception given any proposal that deals with early human history. There is a long tradition of catastrophist ideas, going back to the biblical flood and Plato's story of Atlantis. Philosophically, many people prefer the idea that humans have not had much effect on the planet, either 13,000 years ago or today—better to blame thunderbolts from the gods than to accept responsibility for our stewardship of Earth.

Acknowledgments

I am grateful to Mark Boslough, Clark Chapman, and Alan Harris for many stimulating discussions of the YD impact hypothesis and especially for their insightful and generous suggestions for improving this paper.

References

Alvarez, L.W., W. Alvarez, F. Asaro, and H.V. Michel. 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208: 1095.

Alvarez, W. 1997. T. Rex and the Crater of Doom. Princeton University Press.

Firestone, R.B., and W. Topping. 2001. Terrestrial evidence of a nuclear catastrophe in paleoindian times. Mammoth Trumpet Magazine (March): 9, published by the Center for the Study of the First Americans.

Firestone, R., A. West, and S. Warwick-Smith. 2006. The Cycle of Cosmic Catastrophes: Flood, Fire, and Famine in the History of Civilization. Bear and Company, Rochester, Vermont.

Firestone, R.B., et al. 2007. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proceedings of the National Academy of Sciences 104: 1616.

Geological Society of America (GSA) Annual Meeting (October 18–21, 2009), Portland, Oregon. Relevant oral presentations, with quotes from their abstracts. Paquay et al.: No evidence of extraterrestrial geochemical components at the Bølling-Allerød/Younger Dryas transition. ("Our study discredits the YD impact hypothesis.") Surovelle and Holliday: Non-reproducibility of Younger Dryas extraterrestrial impact results. ("We were unable to reproduce any results of the original Firestone et al. study and find no support for Younger Dryas extraterrestrial impact.") Pinter et al.: Extraterrestrial and terrestrial signatures at the onset of the Younger Dryas. ("Many of the purportedly unique markers at the YD boundary layer were found in most or all other sites and horizons analyzed, often at concentrations much higher than at the YD layer itself.") Holliday and Meltzer: Geoarchaeology of the 12.9 ka impact hypothesis. ("Sites purported to provide direct evidence of the 12.9 ka impact are not well constrained to that time. An ET impact is an unnecessary ‘solution' for an archaeological problem that does not exist.")

Gill, J., J.W. Williams, S.T. Jackson, K.B. Lininger, and G.S. Robinson. 2009. Pleistocene megafaunal collapse, novel plant communities, and enlarged fire regimes in North America. Science 326: 1100.

Kennett, D.J., et al. 2009. Nanodiamonds in the Younger Dryas sediment layer. Science 323: 94.

Kerr, R. 2007. Mammoth-killer impact gets mixed reception from Earth scientists. Science 316: 1264.

———. 2008. Experts find no evidence for a mammoth-killer impact. Science 319: 1331.

———. 2009. Did the mammoth slayer leave a diamond calling card? Science 323: 326.

Powell, J. 1998. Night Comes to the Cretaceous: Dinosaur Extinction and the Transformation of Modern Geology. Freeman.

Plait, P. 2008. Death from the Skies: These Are the Ways the World Will End. Viking Press.

Raup, D.M. 1991. Extinction: Bad Genes or Bad Luck? W.W. Norton.

Signor, P.W., and J.H. Lipps. 1982. Sampling bias, gradual extinction patterns, and catastrophes in the fossil record. In: Geological Implications of Impacts of Large Asteroids and Comets on the Earth, L.T. Silver and P.H. Schultz, editors. Geological Society of America Special Publication 190: 291.

Toon, O.B., K. Zahnle, D. Morrison, R. Turco, and C. Covey. 1997. Environmental perturbations caused by the impacts of asteroids and comets. Reviews of Geophysics 35: 41.

Ward, P.D., W.J. Kennedy, K.G. MacLeod, and J.F. Mount. 1991. Ammonite and inoceramid bivalve extinction patterns in Cretaceous/Tertiary boundary sections of the Biscay region (southwestern France, northern Spain). Geology 19: 1181.

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